Hi Arno
On windows the preferences file should be in AppData\Roaming\MongoDB Compass\AppPreferences\General.json
On Mac ~/Library/Application Support/MongoDB Compass/AppPreferences/Global.json
and on Linux ~/.config/MongoDB Compass/AppPreferences/General.json
You can just open it in a text editor and copy/paste the text. Or have a look and see if you can spot if it is valid/invalid JSON.
On which operating system are you and which version of Compass is this? We released version 1.40.2 that should work around the problem of corrupted preferences.